So I tried your project and experiences the same issues you have. Tried to tinker some things but to no avail.
And then I tried to make a VehicleAdvanced project and made a Spawn vehicle via Server and it works just fine. Here’s the one I made: https://mega.nz/#!bGph3QKC!cgERwypH6oX5FdrZbJ7eY_aKrhL3RlAZ-BUV_4u1c04
I think there’s an issue either on the importing or the Physics setup. Unfortunately I’m not expert on it so I can only suggest if you can somehow compare your project with VehicleAdvanced and see what causes the issue.
EDIT: Also, when I tried to change the Mesh of your vehicle from GC to ThirdPerson, it spawns just fine. So as you believe, its probably on the vehicle physics setup itself.